The present apparatus relates to automotive exhaust systems, and more particularly relates to thermally-activated exhaust treatment devices that are vacuum-insulated and that have expansion joints and supports within the devices designed to accommodate differences in thermal expansion of a “hot” inner housing relative to a “cool” outer housing.
Most vehicle exhaust systems and particularly exhaust systems of vehicles powered by internal combustion engines are equipped with catalytic converters for reducing noxious emissions in exhaust gases. A problem exists in that a large part of tailpipe hydrocarbon emissions occur during the initial cold start phase when the catalytic converter is least effective. Specifically, cold internal combustion engines produce an exhaust having a relatively high concentration of emissions, while “cold” catalytic converters are least able to deal with the emissions because their catalysts are not efficient until they heat up and reach an operating temperature. (See Benson U.S. Pat. No. 5,477,676, col. 1, ln. 48+). One way of improving upon this situation is to keep the catalytic converters hotter for a longer period of time after an engine is shut off, so that the catalytic converter is still hot even if the engine is started hours later after the engine has cooled off.
Vacuum insulation can be a very effective technique to keep the catalytic converters hot for long periods of time because vacuum minimizes heat loss from air/gas convection, leaving only heat loss from radiation and conduction through solids. However, unacceptable amounts of heat loss by radiation and conduction may still occur at locations where an inner housing is supported inside an outer housing of a catalytic converter. Further, it is not easy to consistently and securely support an inner housing within an outer housing without allowing any direct physical contact between the two housings, especially on a production basis. This is because production processes and the manufactured components exhibit variations and have tolerances that make it difficult to maintain a precise and consistent gap between inner and outer housings on all assemblies produced. Further, where a relatively high vacuum is drawn in the space between the inner and outer housings, the problem of maintaining the gap is made significantly more difficult. This is because the existence of the vacuum creates unbalanced forces on the housing sidewalls, and tends to draw and deform the sidewalls of the inner and outer housings toward each other. Further, even if a gap is successfully made that extends completely around the inner housing between the inner and outer housings, differences in thermal expansion can cause contact between the inner and outer housings. The differences in thermal expansion occur because the inner housing is closer to the hot catalytic materials in the catalytic converter, while the outer housing is cooled by the environment. As a result, differences of several millimeters of thermal expansion can occur. Another problem is the abuse that occurs to the catalytic converter when in service under the vehicle, including impacts and shocks from stones and debris and temperature spikes from high speed/load events or engine misfires. On the other hand, any supports that are provided for holding the inner housing in non-contact with the outer housing cannot be so massive and large as to create a thermally conductive path that defeats the effectiveness of the other insulating features. Thus, designing a support system that supports the inner housing in the outer housing in a reliable and stable manner and that at all times prevents contact, yet that does not itself provide unwanted conduction, is not an insignificant or easily solved problem.
Some insulating arrangements include a fibrous insulation that supports inner and outer tubes along their length, with the fibrous insulation providing separation by being physically positioned between the inner and outer tubes at all locations. (See Bainbridge U.S. Pat. No. 5,163,289). This reliably maintains spacing between the inner and outer tubes, but is not satisfactory since it can result in significant heat transfer along the fibers from the inner tubes to the outer tubes. Further, unless there are many fibers or larger fibers with strength, the fibers will crush and not provide a satisfactory insulating value while in service. Still further, fibrous insulation is not inexpensive.
Thermally-activated exhaust treatment devices also include particulate traps for capturing and treating particulate emissions, each as carbon particles and soot from diesel engines. Particulate traps work best at elevated temperature. Particulate traps are least effective at cold starts which is when the problem of carbon particulate emissions and creation of soot is the greatest in diesel engines. Accordingly, there are significant advantages to be achieved in particulate traps by vacuum-insulating them to conserve and hold their temperatures longer on engine shut off.
Accordingly, an exhaust treatment device is desired solving the aforementioned problems and offering the aforementioned advantages, with the structure including supports that provide for minimal thermal conductivity, long service life, and facilitate manufacture.